Communication systems, as well as other signal processing arrangements, are, more and more, being created utilizing optical signals requiring the use of optical fibers and the numerous components associated therewith, or are being converted to such use. The tremendous bandwidths characterizing optical signal transmission make the use of such systems the preferred mode for signal transmission. However, despite the obvious advantages from the use of optical signals, such transmission gives rise to problems that are unique to light wave signal manipulation, problems which generally are not present in lower frequency signal transmission utilizing conductive wires. For example, optical switching, multiplexing, and demultiplexing are all operations, along with others, that present problems. Such operations have, in general, required a concatenation of discrete optical components, and, as a result, suffer from increased bulkiness and reduced system reliability. As a consequence, much effort has been directed at reducing the number of components by combining their operations on a single, monolithic chip which generally comprises a thin film, compact planar optical circuit. Properly designed chips can greatly improve circuit or signal transmission performance while, at the same time, insuring greatly increased reliability and an advantageous reduction in the number of discrete components in the circuit.
One example of such a monolithic device is the dense wave division multiplexer (DWDM) of the type shown, for example, in U.S. Pat. No. 5,136,671 of Dragone, the disclosure of which is herein incorporated by reference. Such a device is useful where a large number of transmission channels must be crowded into a narrow bandwidth window. Such narrow windows can result, for example, from the use of erbium doped optical amplifiers, which are widely used today, but which can severely limit the usable bandwidth. In order to accommodate many channels in the narrow window, they must be closely spaced in wavelength, such as, for example, successive wavelengths differing by 0.8 nm or 1.6 nm. By use of a new technology referred to as OASIC (optical application specific integrated circuits), thin film planar optical circuits can be formed to produce such a DWDM as discussed, on a single wafer or chip. Such a chip generally comprises a thin silicon wafer upon which a low refractive index silica glass lower cladding is deposited. A high index core layer is then deposited, patterned, and etched to form the optical waveguides, and then an upper cladding is deposited. Wafers for a variety of functions can be produced using the OASIC technology, however, the remainder of the discussion will be directed to the DWDM in the interest of simplicity and consistency. It should be understood that these other types of integrated circuits are by no means intended to be excluded.
One of the problems arising from the use of some OASIC devices, particularly the arrayed waveguide gratings in a DWDM, is their sensitivity to temperature changes, and to physical stresses which impair their reliability. For example, in the DWDM, because the operating wavelengths of the several individual channels differ by such a small degree, any expansion or contraction or bending due to temperature fluctuations will degrade the optical performance and, in the extreme, cause circuit failure. Likewise, temperature fluctuations less than 1.degree. C. may cause degradation or failure. It has been found that degradation or failure can generally be prevented and reliability of the circuit insured if the temperature of the device is maintained at a predetermined temperature in a range of 75.degree. C. to 90.degree. C. This maintenance temperature, specific to the individual circuit, must be controlled to within a few degrees Celsius even though the ambient temperature may vary from, for example, 0.degree. C. to 70.degree. C. Thus, some sort of protective housing must be provided for the wafer, i.e., circuit.
Maintaining various types of electronic devices at an even temperature by housing them in sealed containers is well known in the art. In U.S. Pat. No. 4,968,121 of Brusselbach et al. there is shown such a housing for a crystal which comprises an oven having walls of thermally conductive material and an inner chamber within the oven which contains the crystal and maintains it in thermal contact with the walls of the inner chamber. Means are provided for heating both the oven walls and the crystal. Such an arrangement is both complicated and bulky, and, apparently, unsuitable for maintaining an IC wafer at a constant temperature with an economy of components and structure.
In any housing for maintaining a substantially constant temperature in at least a portion thereof, each component within the housing, as well as the housing itself, is subject to deleterious temperature effects during operation. Thus, expansion or contraction of the housing itself due to temperature changes can affect at least some of the components within the housing, to the detriment of the overall operation.